Satellite Communication

1
Satellite Communication

• Lecture 7
• Two-way Interactive Communication for Fixed and
  Mobile Users

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Overview

• Two-way Interactive Communication for Fixed and
  Mobile Users
• Introduction to VSAT Networks for Interactive
  Applications
• Principle of Protocol Layering
• Protocols supported by VSAT Networks
• Satellite Point to Point Connectivity
• Satellite Point to Multipoint Connectivity
• VSAT Star Networks
• Applications
• Architecture
• Personal Computer Integration with the VSAT
• Operation of Multiple Access Protocols in VSATs
• Mobile Satellite Services

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Introduction

• VSAT networks are composed of low-cost Earth
  stations for use in a wide variety of
  telecommunications applications.
• Unlike the point-to-multipoint systems VSATs are
  two-way communications installations designed to
  achieve interactivity over the satellite
• Interconnection with various terrestrial networks
  is also a feature.
• Internet has taken over the role of the common
  structure for integrating data communications for
  the majority of applications in information
  technology (IT).
• This has rationalized the field to the point that
  a single protocol and interface standard provide
  almost all of what an organization needs.

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Introduction (Contd.)

• The same approach works equally well for
  individuals and the small office/home office
  (SOHO) environment.
• Satellite communications technology has adapted
  to this new world as well.
• Oddly, it was not until the early 1980s that
  satellite systems found a direct place in this
  expanding field.
• The overriding principle of the VSAT is that it
  is a small bidirectional Earth station that
  delivers integrated data, voice, and video
  services within a package that is often cost
  justified when compared to terrestrial
  alternatives.

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Introduction (Contd.)

• Today, terrestrial copper, fiber lines, data
  routing and switching in conjunction with VSATs
  provide a fast and effective mix to advance the
  competitive strategy of many medium to large
  businesses.
• VSAT networks also address the needs of small
  businesses and individuals.
• The three classic architectures for IT networks
  are host-based processing (utilizing centralized
  large-scale computers like mainframes),
  peer-to-peer networks (usually employing
  minicomputers or large servers that are deployed
  at different locations to serve local
  requirements), and client/server networks (which
  tie together personal computers, servers, and
  peripherals using LANs and WANs).

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Introduction (Contd.)

• The three classic architectures for IT networks
  are
• Host-based processing (utilizing centralized
  large-scale computers like mainframes),
• Peer-to-peer networks (usually employing
  minicomputers or large servers that are deployed
  at different locations to serve local
  requirements), and
• Client/Server networks (which tie together
  personal computers, servers, and peripherals
  using LANs and WANs).
• VSAT networks now address the needs of small
  businesses and individuals in all these areas.

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Principle of Protocol Layering

• Modern data communications theory and practice is
  literally built upon the concept of protocol
  layering, where the most basic transmission
  requirement is at the bottom and more complex and
  sophisticated features are added one on top of
  each other.
• While this concept is abstract, it is important
  to understanding how the data in a network is
  assembled, processed, and reliably transferred
  between sender and receiver.

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Principle of Protocol Layering

• It has evolved over decades of telecommunications
  development, beginning with the most simple voice
  radiotelephone network, through networks that
  support national air defense, applied in business
  for large-scale data processing, and evolved into
  the pervasive structure of the Internet.
• The layering concept is embodied in the Open
  Systems Interconnection (OSI) model shown in
  Figure 8.1 and contained in relevant standards of
  the International Standards Organization (ISO)
  and the ITU-Telecommunication Sector (ITU-T).

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OSI and TCP/IP (DARPA) Model
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Principle of Protocol Layering

• Layer 1, physical provides the mechanism for
  transmitting raw bits over the communication
  medium (e.g., fiber, wireless, and satellite).
• It specifies the functional, electrical, and
  procedural characteristics such as signal timing,
  voltage levels, connector type, and use of pins.
• The familiar RS-232 connector definition is a
  good example of the physical layer.
• A way to look at this is that the physical layer
  takes the raw bit stream at the sending end and
  introduces it to the network.
• All together, most of the investment in a
  satellite network is at the physical layer.

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Principle of Protocol Layering

• Layer 2, data link provides for the transfer of
  data between adjacent nodes or connection points
  either by a dedicated point-to-point line (e.g.,
  a T1 private line or a satellite duplex link) or
  a medium capable of shared bandwidth (e.g., an
  Ethernet cable or satellite TDMA channel).
• The link layer can offer a one-to-one connection
  (the most common approach) or one-to-many
  delivery (associated with broadcast or
  multicast).
• Layer 3, network responsible for routing
  information from end to end within the network,
  which would consist of multiple data link paths.
• This may involve decisions about the most
  effective route through the point-to-point links
  that comprise the network.
• A VSAT network may serve as one of these links
  and hence would have to interface properly with
  the network layer.

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Principle of Protocol Layering

• Layer 4, transport provides another level of
  assurance that the information will properly
  traverse the network, fromend user to end user.
• Two services are commonly available
  connectionless, which transfers packets of data,
  one at a time and connection oriented, where a
  virtual circuit is first established before
  sending multiple packets that make up the entire
  conversation.
• The familiar TCP layer of TCP/IP provides a
  connection-oriented service to computer
  applications.
• Layer 5, session somewhat more complicated than
  layers 3 and 4 but provided to instill yet
  greater degrees of reliability and convenience of
  interface to applications.
• It manages the data exchange between computer
  systems in an orderly fashion to provide
  full-duplex or half-duplex conversations.
• One important service is that of reestablishing
  the connection in the event that the transport
  layer is interrupted for some reason.

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Principle of Protocol Layering

• Layer 6, presentation provides syntactic and
  semantic services to the application layer above.
  
• The presentation layer is inserted to resolve the
  complexities between transport/network layers and
  the more simplistic needs of the actual
  application that employs the network in the first
  place.
• Some specialized services like encryption and
  data structure definition are considered to be
  part of the presentation layer.
• Interaction of the presentation layer with
  elements of a satellite network may cause
  incompatibility, requiring additional processing
  to be performed in Earth station equipment or
  user terminals.

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Principle of Protocol Layering

• Layer 7, application includes the actual data
  communication applications that are common in
  open systems, such as file transfer, virtual
  terminal, e-mail, and remote database access.
• We refer to these as applications because they
  include not only the protocol elements that
  support specific types of information but also
  features and facilities that ultimately interact
  with the end user.
• Most non-expert users will not use the
  application layer directly, instead relying on
  specialized software within the computer to
  improve the interface and functionality.
• For example, most subscribers to on-line
  information services use the e-mail package
  supplied by the provider.
• This package, in turn, will engage layer 7 e-mail
  services to do the actual function of sending and
  receiving message traffic.

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Principle of Protocol Layering

• The VSAT network is ideal for centralized
  computer networksthat is, those that employ a
  host computer.
• The majority of such installations are assembled
  from standard computer components supplied in the
  United States by Dell, HP, IBM, and Sun
  Microsystems
• Major European and Japanese suppliers like Bull,
  Siemens, Fujitsu, and Hitachi are in this market
  as well.
• Following the layering concept, each computing or
  terminal device in the network has a unique
  address that identifies that device at the
  specified layer.
• Some examples of addressing schemes are given in
  Table 8.1

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Principle of Protocol Layering
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Protocols supported by VSAT Networks

• A summary of the protocols in general use and
  their support over typical VSAT networks is
  provided in Table 8.2.
• When first introduced in the 1980s, VSATs played
  heavily on the traditional IBM proprietary
  protocol, Systems Network Architecture (SNA),
  which followed the same centralized approach as
  the VSAT star network.
• While still in existence in some legacy
  environments, it has been replaced with the more
  open Internet Protocol suite (TCP/IP).
• TCP/IP has its shortcomings, which are being
  addressed by standards bodies and major vendors
  like Cisco.
• Employing TCP/IP in a private network is very
  straightforward and is well within the means of
  any organization or individual.

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Protocols supported by VSAT Networks
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Protocols supported by VSAT Networks

• However, the complexity comes when an
  organization wishes to interconnect with the
  global Internet and with other organizations.
• This is due to the somewhat complex nature of
  routing protocols like the Border Gateway
  Protocol (BGP) and a new scheme called Multi
  Protocol Label Switching (MPLS).
• Frame Relay has been popular in WANs for more
  than a decade, thanks to its ease of interface at
  the router and availability in (and between)
  major countries.
• It is capable of near-real-time transfer and can
  support voice services. With access speeds
  generally available at 2 Mbps or less.
• Satellite provision of Frame Relay has been
  limited to point-to-point circuits as the
  protocol is not directly supported in VSATs
  currently on the market.
• The best approach would be to use TCP/IP in lieu
  of Frame Relay when VSAT links are interfaced at
  the router.

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Satellite Point to Point Connectivity

• The first satellite networks to be implemented
  were employed for point-to-point connectivity to
  complement the cross-country microwave and
  undersea cable links of the time.
• This topology remains an effective means of
  transferring information with minimum delay
  between pairs of points.
• As illustrated in Figure 8.2, node 1 in a
  point-to-point service conducts a full-duplex
  conversation with node 2 (shown with heavy
  arrows), and node 3 does likewise with node 4
  (shown with broken arrows).
• For applications such as Fixed Telephony
  Satellite Services, Point-to-point connectivity
  between node 1 and node 3 can be changed on
  demand.

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Satellite Point to Point Connectivity
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Satellite Point to Multipoint Connectivity

• The point-to-multipoint connectivity is
  illustrated in Figure 8.4.
• The thick, shaded arrows represent the digital
  broadcast outroute from the hub to the remote
  nodes (other acceptable terms for the hub
  transmitted signal are outbound, forward, and
  downstream).
• It contains all hub-originated data to be
  delivered to the VSATs throughout the network.
• This transmission is received by all remotes
  within the satellite footprint however, it would
  typically contain address information that allows
  only the desired remotes to select the
  information destined for them.
• The thin lines represent the inroutes from the
  individual remote nodes (likewise, acceptable
  terms include inbound, return, and
  upstream).

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Satellite Point to Multipoint Connectivity
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VSAT Star Networks

• Organizations employ VSATs primarily as
  replacements for terrestrial data networks using
  private lines in a variety of applications,
  including retailing, postal and package delivery,
  automobile sales and service, banking and
  finance, travel and lodging, and government
  administration and security.
• Perhaps the first major installation was for
  Wal-Mart, the leading U.S. retailer with stores
  throughout the United States and other locations
  around the world.
• Today, there are more than 250,000 two-way VSATs
  installed in the United States and over 600,000
  worldwide.
• Not included is the consumer VSAT designed to
  provide Internet access

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VSAT Star Networks
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VSAT Star Networks

• VSAT technology should only be used as a
  supplement to high-quality digital fiber optic
  and wireless networks of the world. In fact, the
  best strategy is often to complement the
  terrestrial network infrastructure with VSATs so
  as to achieve an optimum and reliable mix.
• For example, a European company needing to
  connect only five domestic locations to a data
  center would find that conventional VSATs may not
  be cost-effective.
• Likewise, a large industrial organization that
  needs high-capacity links between major sites is
  not a candidate for existing VSATs.
• This would clearly be a better application for
  fiber optic links, if that were feasible, or
  point-to-point satellite links.

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VSAT Star Networks Applications

• Many centralized companies build their IT systems
  around the host computer that is located at the
  headquarters or outsourced hosting facility.
• This is an ideal starting point for VSAT network
  adoption since it is centralized.
• Table 8.4 provides a listing of popular IT
  applications now provided over enterprise VSAT
  networks.
• Examples include
• Retail MarketingWal-Mart and JD Group
• AutomotiveDaimler-Chrysler and Toyota
• US Postal Services
• Retail BankingBanamex
• The architecture of the typical VSAT star network
  is provided in Figure 8.6

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VSAT Star Networks Applications

• The architecture of the typical VSAT star network
  is provided in Figure 8.6 that depicts how the
  user connects computers, PCs and other terminals,
  PBX and telephone systems, and video equipment
  used in private broadcasting.
• The hub of the star is shown on the right in the
  form of a complete Earth station facility with a
  relatively large antenna (typically 4.7m at
  Ku-band and 9m at C-band).
• The most common implementations of the star
  network use TDMon the outroute and TDMA as well
  as a derivative called ALOHA on the inroute.

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VSAT Star Networks Architecture
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Personal Computer Integration with the VSAT

• The PC is the ideal direct user interface with
  the VSAT in applications where on-line
  information delivery is required.
• Typical telephone networks have a real throughput
  of about 40 Kbps lower rates are common in areas
  where line quality is poor.
• This was once adequate for applications such as
  on-line service connection, dial-up terminal
  access to e-mail, and fax.
• With the growth of the World Wide Web and the
  increasing demand for the transfer of large files
  for graphics, database, and engineering
  applications, the analog telephone network ceases
  to be adequate.
• The marketplace is provided with VSAT networks
  that have typical inbound throughputs in the
  range of 128 Kbps to 2Mbps.

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Personal Computer Integration with the VSAT

• An example of using the point-to-multipoint
  feature and the PC is shown in Figure 8.9.
• The data files or streams are uplinked from a hub
  Earth station at the right.
• A public ISP or content delivery network service
  would own and operate the hub.
• Information is delivered to the hub over backhaul
  circuits from one or more servers or other
  information sources (e.g., a stock market
  ticker).
• Subscribers purchase and install a receive-only
  VSAT, which need only receive the high-speed
  forward link broadcast from the hub.

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Personal Computer Integration with the VSAT
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Operation of Multiple Access Protocols in VSATs

• The inbound channel is shared by multiple VSATs
  that transmit their data in bursts.
• Two basic multiple access methods are used for
  this purpose
• TDMA
• ALOHA

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Operation of Multiple Access Protocols - TDMA

• An example of a TDMA burst time frame lasting
  about 45 ms is provided in Figure 9.4.
• As applied to the inbound channel, the
  transmissions from the VSATs are coordinated and
  highly synchronized so as to prevent overlap and
  a resulting loss of information.
• Each station (numbered 1 through 10) is allotted
  a fixed interval of time in which to transmit
  data.
• The frame repeats every 45 ms, producing an
  average delay per inbound channel burst due to
  multiple access of 45/2 22.5ms.
• Obviously, the shorter the frame, the less the
  average delay.

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Operation of Multiple Access Protocols - TDMA
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Operation of Multiple Access Protocols - ALOHA

• Another approach for separating the inbound
  channel transmissions in time is the ALOHA
  protocol.
• The scheme is simpler in that the transmissions
  are uncoordinated however, the complexity occurs
  because there are occasional overlaps that result
  in lost communication.
• This is overcome by retransmissions from the
  affected VSATs.
• For example, a slotted ALOHA channel with three
  users is shown in Figure 9.5.
• Slotting refers to requiring that the ALOHA
  packets fall within timed periods, indicated by
  the vertical lines.
• The upper three horizontal lines represent three
  VSAT uplinks the bottom timeline depicts the
  downlink showing how the ALOHA packets appear
  after passing through the satellite repeater.

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Operation of Multiple Access Protocols - ALOHA

• Each VSAT remains in an idle state until there is
  data to be transmitted.
• Lets assume that VSAT 1 is the first to need the
  channel and so transmits the block of data
  without waiting.
• VSAT 2 transmits next, independently of what
  happens at users 1 and 3.
• From the downlink timeline, we see that VSAT 1
  and VSAT 2 do not overlap and hence get through
  in the clear.
• The next packets from VSATs 1 and 3 have reached
  the satellite at approximately the same time and
  so have produced a collision.
• In the event of such a time overlap, the signals
  jameach other and the information is lost
  (indicated by the presence of a dark block in the
  downlink).
• Neither packet is received at the huba condition
  that is inferred by these VSATs because of
  non-acknowledgment by the hub over the outbound
  channel.

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Operation of Multiple Access Protocols - ALOHA

• The way that packets are ultimately transferred
  is through automatic retransmissions, as shown at
  the ends of the curved arrows in Figure 9.5.
• The delay between the original and retransmitted
  packets is selected randomly by each VSAT to
  reduce the possibility of a second collision.
• The result of this protocol is that the delay is
  as small as it can possibly be for a packet that
  does not experience a collision.
• For one that does, the delay is lengthy since it
  includes at least two round-trip delays plus the
  delays of the random offset as well as from
  processing within the hub and VSAT.
• In an acceptable operating situation, only 1 in
  10 ALOHA packets will experience a collision.

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Operation of Multiple Access Protocols - ALOHA
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Mobile Satellite Services

• Historically, GEO satellites have provided most
  of the MSS capabilities, in terms of land, sea,
  and air.
• The economy and simplicity of a single satellite
  along with the ability to use fixed antennas on
  the ground have allowed GEO to become most suited
  for these applications.
• In addition to the global capability of Inmarsat,
  a number of GEOMSS networks capable of serving
  handheld satellite telephones are in service.
• The major benefit of the lower orbits is reduced
  time delay for voice services.
• This factor is very important in terrestrial
  telephone networks, particularly with
  high-quality transmission as provided through
  fiber optic technology.
• Table 11.3 provides a summary of key attributes
  of LEO, MEO and GEO Satellites.

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Mobile Satellite Services
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Mobile Satellite Services
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Mobile Satellite Services
Figure 11.9 A selection of user terminal
equipment for use with Mobile Satellite Services
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Mobile Satellite Services
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QA

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